1. Technical Field
The present disclosure relates to a multi-level shifter for ultrasound applications and a transmit/receive channel for ultrasound applications using said level shifter.
Particularly, the present disclosure relates to a high-voltage multi-level shifter for ultrasound applications coupled between a connecting terminal and an output terminal of a transmit channel, of the type comprising at least one first and one second communication transistors, coupled in series with each other between the connecting terminal and the output terminal.
The disclosure also relates to a transmission channel of the type comprising at least one high-voltage multi-level shifter as described above, e.g., for use in an ultrasound imaging or ultrasonography.
2. Description of the Related Art
An ultrasound or sonographic machine is known to be a medical diagnostic testing system that uses ultrasonic waves or ultrasounds and is based on the principle of ultrasound transmission and echo emission analysis and is widely used in internal medicine, surgery and radiology.
Typically used ultrasounds range from 2 to 20 MHz.
Frequency is selected considering that higher frequencies have a greater image resolving power, but penetrate to a shallower depth in the individual under examination.
These ultrasounds are typically generated by a piezoceramic crystal in a probe that in kept in direct contact with the skin of the individual with the interposition of an appropriate gel (which is adapted to eliminate air between the probe and the skin of the individual, thereby allowing ultrasounds to penetrate the anatomic region being examined).
The probe can collect a return signal, or echo, which is appropriately processed by a computer and displayed on a monitor. Particularly, ultrasounds that reach an acoustic impedance variation point, such as an internal organ, are partially reflected and the percentage reflection provides information about impedance differences between the penetrated tissues.
The time that an ultrasonic wave takes to run its path of propagation, reflection and return is provided to the computer, which calculates the depth from which the echo is emitted, and thus identifies the boundary surface between the penetrated tissues (which corresponds to the acoustic impedance variation point and hence to the depth from which the echo is emitted).
A typical transmit/receive or TX channel that is used in these applications is schematically shown in FIG. 1 and generally designated by numeral 1.
Particularly, the transmit/receive channel 1 comprises a high voltage multi-level shifter 2 of the type comprising a branch 4 coupled between a first terminal HVP connected to a positive voltage, and a second terminal HVM, connected to a negative voltage.
In the illustrated example, the shifter 2 allows switching between two levels, i.e., the high level, corresponding to the voltage of the first terminal HVP and the low level HVM.
Typical values for the terminal HVP connected to a positive voltage range from 5V to 100V, whereas typical values for the terminal HVM connected to a negative voltage range from −5V to −100V.
The voltage of the output terminal HVout of the level shifter 2 is clamped by a clamping block 5 to a reference voltage, in this example the ground voltage GND.
The clamping block 5 is substantially a high-voltage switch coupled between said output terminal HVout of the level translator 2 and said ground voltage GND, and controlled by a first control signal INC.
The output terminal HVout, which corresponds to a first output terminal of the transmission channel 1, is connected to a connection terminal Xdcr for the piezoelectric transducer to be controlled by the transmission channel 1.
Conveniently, an anti-noise block 3, comprising two anti-parallel diodes, is coupled between the output terminal HVout of the level shifter 2 and the connection terminal Xdcr.
The anti-noise block 3, known as anti-noise diodes, allows the stray capacitances of the half-bridge of the level shifter 2 to be isolated from the connection terminal Xdcr during reception by the transmission channel 1.
A transmit/receive (T/R) switch 6 or transmit/receive switch is coupled between the connecting terminal Xdcr and a low-voltage output terminal LVout of the transmission channel 1. During reception by the transmission channel 1, the receive switch 6 is actuated and transmits the received signal to the low-voltage output terminal LVout.
The low-voltage output terminal LVout is connected to an amplifier LNA which allows amplification of the signals or echoes received by the piezoelectric transducer after pulse transmission.
The receive signal, after appropriate processing, will allow an image to be displayed on a screen, not shown.
It shall be noted that the receive switch 6 is of the high-voltage type even if the receive signal is generally a low-voltage signal, because the piezoelectric transducer connected to the transmission channel 1 detects small return echoes of ultrasound pulse signals.
Nevertheless, the receive switch 6 should fulfill two apparently opposite specifications: it should be of high-voltage type during transmission by the transmission channel 1, in which the level shifter 2 has times ranging from tens to hundreds of nanoseconds, and it should operate at low voltages during reception, where reception may take a few hundreds of microseconds.
Furthermore, the receive switch 6 and the clamping block 5 are generally formed as two separate chips, particularly located in a receive chip ad a transmit chip or, if present in the same chip, they are separately formed to properly fulfill their respective specifications.
More in detail, the high-voltage shifter 2 comprises a first branch, having a first switching transistor M1 and a second switching transistor M2, which are coupled with each other between the first higher voltage reference terminal HVP and the first lower voltage reference terminal HVM. The first and second transistors M1 and M2 have respective control terminals connected to and controlled by first DRM1 and second DRM2 input drivers, and the respective drain terminals connected together.
It shall be noted that the first switching transistor M1 is a high-voltage P-channel MOS transistor (HV PMOS), and the second switching transistor M2 is a high-voltage N-channel MOS transistor (HV NMOS).
Therefore, in classical ultrasonic solutions, the level shifter is obtained using asymmetric output stages (NMOS and PMOS) which inevitably generate a second harmonic distortion.
Thus, when the circuit 1 is switched from a high-voltage level (e.g., HVP) to a low-voltage level (e.g., HVM) or vice versa, the shifter 2, which is constructed with MOS transistors of different types (NMOS vs PMOS), has different transitions to the output terminal HVout depending on whether the shifter is switched to a high-voltage value or a low-voltage value.
This asymmetry in the rising or falling edge of the voltage signal to the terminal HVout causes a second harmonic component to be introduced into the emitted signal, and thus disturb later second-harmonic analysis on the reflected echo.
Generally, this asymmetry may be minimized according to the current/voltage characteristics of the two NMOS and PMOS transistors, by having them operate in appropriate range of operating conditions (such as output load and operating voltage).
Nevertheless, this optimization is not stable and accurate and especially, with changing operating conditions, it may lead to a considerable degradation of performances, possibly to 10 db lower attenuation of the transmitted second harmonic component.
This introduced asymmetry particularly affects the percentage of the reflected acoustic signal, which carries information about the difference in impedance between the penetrated tissues.
This second harmonic distortion is tolerated (in non-high-quality applications), when it is attenuated by about 30-40 db with respect to the value of the carrier of the generated acoustic signal.
Nevertheless, due to this distortion, the images of the region to be observed are generated with a resolution that is lower than the one that might be obtained without such asymmetry.